[PATCH] KVM: x86: Add module parameter for lapic periodic timer limit

[Jan Kiszka]

Certain guests, specifically RTOSes, request faster periodic timers than
what we allow by default. Add a module parameter to adjust the limit for
non-standard setups. Also add a rate-limited warning in case the guest
requested more.

Signed-off-by: Jan Kiszka <***@siemens.com>
---
arch/x86/kvm/lapic.c | 15 +++++++++++++--
1 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 4b53b81..2fb20ca 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -68,6 +68,9 @@
#define VEC_POS(v) ((v) & (32 - 1))
#define REG_POS(v) (((v) >> 5) << 4)

+static unsigned int min_timer_period_us = 500;
+module_param(min_timer_period_us, uint, S_IRUGO | S_IWUSR);
+
static inline u32 apic_get_reg(struct kvm_lapic *apic, int reg_off)
{
return *((u32 *) (apic->regs + reg_off));
@@ -677,8 +680,16 @@ static void start_apic_timer(struct kvm_lapic *apic)
* scheduler.
*/
if (apic_lvtt_period(apic)) {
- if (apic->lapic_timer.period < NSEC_PER_MSEC/2)
- apic->lapic_timer.period = NSEC_PER_MSEC/2;
+ s64 min_period = min_timer_period_us * 1000LL;
+
+ if (apic->lapic_timer.period < min_period) {
+ pr_info_ratelimited(
+ "kvm: vcpu %i: requested %lld ns "
+ "lapic timer period limited to %lld ns\n",
+ apic->vcpu->vcpu_id, apic->lapic_timer.period,
+ min_period);
+ apic->lapic_timer.period = min_period;
+ }
}

hrtimer_start(&apic->lapic_timer.timer,

[Marcelo Tosatti]

Post by Jan Kiszka
Certain guests, specifically RTOSes, request faster periodic timers than
what we allow by default. Add a module parameter to adjust the limit for
non-standard setups. Also add a rate-limited warning in case the guest
requested more.
---
arch/x86/kvm/lapic.c | 15 +++++++++++++--
1 files changed, 13 insertions(+), 2 deletions(-)

Applied, thanks.

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